The transcutaneous (TC) non-invasive measurement of the partial pressure of blood gases such as carbon dioxide (pCO2) or oxygen (pO2) by means of a TC electrochemical sensor applied to the skin has been described in many publications as a useful tool in a number of clinical situations. An electrochemical sensor for such use is known from e.g. U.S. Pat. No. 6,654,622 B1. Incorporated in and opening into the measuring surface of such electrochemical sensor are both an electrode for measuring pCO2 and an electrode for measuring pO2 as well as a reference electrode.
TC electrochemical sensors comprise among others a membrane system covering the surface of the sensor and an electrolyte solution placed between the membrane and the measuring surface. The electrolyte solution creates a bridge for electrons between the pCO2 electrode and the pO2 electrode of the sensor.
In the most common TC electrochemical sensors, the pCO2 electrode comprises a pH glass electrode and the pO2 electrode comprises a cathode. e.g. a Pt cathode. However, it has long been known that during use of the TC sensor a growth of metallic material (dendrites) builds up on the measuring surface and around the measuring surface of the cathode. The dendrite material origins from metals parts of the sensor in the sensor, thus usually silver, but also gold in minor amounts have seen to appear. The growing of dendrites means that the surface of the cathode increases over time. This increase is found to be not linear but of a higher degree order. The increase of the cathode sensor area increases the sensitivity of the pO2 electrode. Once a predefined sensitivity limit is reached, the sensor is not capable to be calibrated and has to be refurbished. Furthermore, an increased sensitivity results in a linearity deviation, which limits the sensor accuracy on the extreme measurement ranges.
To overcome the problems of dendrites on the Pt cathode surface, the sensor measuring surface has to be cleaned from time to time. The user may be instructed to clean the sensor from time to time, depending on the usage of the sensor or alternatively an upper sensitivity threshold may be defined, such that when this sensitivity threshold is reached, the user is instructed to clean the sensor independently of when the sensor was cleaned the last time. Manuals have instructed the user to thoroughly clean the sensor, but as the dendrites on a Pt cathode are too small to be seen by the human eye, it has been difficult for the user to understand what thorough cleaning meant. If the sensor is not cleaned properly this may result in failure of calibration of the sensor. As the dendrites stick to the Pt cathode and cannot just be “washed away” with a wet cloth (since silver and gold are not soluble in any cleaning agents), it is necessary to scrub or grind the dendrites of the cathode in a so-called dry cleaning process wherein the dry measuring surface is scrubbed with a dry cloth.
If the sensor is not cleaned sufficiently by the user, so as to remove the dendrites, the time before next cleaning decreases and as the growth increases to a higher degree, the time between cleaning consequently fails increasingly fast. This is of course inconvenient and time consuming for the user and as every cleaning of the sensor requires replacement of the membrane system and electrolyte solution it also requires use of extra membrane systems/electrolyte solution. Furthermore, it leads to shorter life time of the sensor, and if not dealt with, makes the measurements unreliable.
One way to overcome the frequent cleaning and reduced sensor life time could be to accept the higher sensitivity which would decrease the reliability of the measured partial pressure of oxygen, due to an increased linearity deviation of the sensor. This would limit the measurement in the extreme measurement range as e.g. (1) patients in hyperbaric chamber or (2) preterm neonates with very low pO2 values due to insufficient oxygen titration, or (3) patients with reduced blood perfusion.
Another solution provided in the field has been to provide a special tool for the dry cleaning step wherein the sensor is placed inside a tool, where it sits firmly and the tool is closed firm around the sensor. The user than turns a knob on the tool whereby rotating disks clean the sensor. This has the advantage that it is a standard cleaning process. It provides a cleaning method better than a very poor and slobby manual cleaning using a cloth, but with the disadvantage of an extra tool necessary for the user and with the risk of “over-cleaning” (destroying the sensor surface).
The object of the invention is to provide a method for monitoring the cleaning progress when the user cleans the sensor and further to provide feedback to the user about the cleaning progress of the sensor. The method provides the user of TC electrochemical sensors with means for determining how well the cathode has been cleaned, or how much more cleaning is necessary before the cathode is clean.